1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an array substrate for use in an in-plane switching mode liquid crystal display device (IPS-LCD).
2. Description of the Related Art
A liquid crystal display device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. Liquid crystal molecules have a definite alignment direction as a result of their long, thin shapes. That alignment direction can be controlled by an applied electric field. In other words, as an applied electric field changes, so does the alignment of the liquid crystal molecules. Due to the optical anisotropy, the refraction of incident light depends on the alignment direction of the liquid crystal molecules. Thus, by properly controlling an applied electric field, a desired light image can be produced.
Of the different types of known LCDs, active matrix LCDs (AM-LCDs), which have thin film transistors and pixel electrodes arranged in a matrix form, are the subject of significant research and development because of their high resolution and superiority in displaying moving images.
LCD devices have wide application in office automation (OA) equipment and video units because they are light and thin and have low power consumption characteristics. The typical liquid crystal display (LCD) panel has an upper substrate, a lower substrate and a liquid crystal layer interposed therebetween. The upper substrate, commonly referred to as a color filter substrate, usually includes a common electrode and color filters. The lower substrate, commonly referred to as an array substrate, includes switching elements, such as thin film transistors (TFTs) and pixel electrodes.
As previously described, LCD device operation is based on the principle that the alignment direction of the liquid crystal molecules is dependent upon an electric field applied between the common electrode and the pixel electrode. Thus, the alignment direction of the liquid crystal molecules is controlled by the application of an electric field to the liquid crystal layer. When the alignment direction of the liquid crystal molecules is properly adjusted, incident light is refracted along the alignment direction to display image data. The liquid crystal molecules function as an optical modulation element having variable optical characteristics that depend upon polarity of the applied voltage.
In a conventional LCD device, since the pixel and common electrodes are positioned on the lower and upper substrates, respectively, the electric field induced between them is perpendicular to the lower and upper substrates. However, the conventional LCD devices having the longitudinal electric field have a drawback in that they have a very narrow viewing angle. In order to solve the problem of narrow viewing angle, in-plane switching liquid crystal display (IPS-LCD) devices have been proposed. The IPS-LCD devices typically include a lower substrate where a plurality of pixel electrodes and common electrodes are disposed, an upper substrate having no electrode, and a liquid crystal interposed between the upper and lower substrates. A detailed explanation about the lower substrate (i.e., array substrate) of the IPS-LCD device will be provided referring to figures.
FIG. 1 is a schematic plan view illustrating one pixel of an array substrate of an in-plane switching mode liquid crystal display (IPS-LCD) device according to a related art. As shown, a gate line 12 is transversely disposed on a substrate 10. A common line 16 is spaced apart from and disposed parallel with the gate line 12. Data lines 24 that are spaced apart from each other are disposed across and substantially perpendicular to the gate and common lines 12 and 16. A pair of data lines 24 and a pair of gate and common lines 12 and 16 define a pixel region P.
A switching device, i.e., a thin film transistor T, is positioned near the crossing of the gate and data lines 12 and 24. As shown in FIG. 1, the thin film transistor T includes a gate electrode 14, an active layer 20, a source electrode 26 and a drain electrode 28. The gate electrode 14 is a portion of the gate line 12 near the data line 24, and the source electrode 26 protrudes from the data line 24 over the gate electrode 14. Namely, the portion of the gate line 12 near the data line 24 acts as a gate electrode and the source electrode 26 is electrically connected to the data line 24. The drain electrode 28 is spaced apart from the source electrode 26 and overlaps a portion of the gate electrode 14. The active layer 20 is located above the gate electrode 14 and under the source and drain electrodes 26 and 28. Thus, the source electrode 26 and the drain electrode 28 overlap opposite portions of the active layer 20, respectively.
A pixel electrode 30 connected to the drain electrode 28 is disposed in the pixel region P. A common electrode 17 that is connected to the common line 16 is also disposed in the pixel region P. The pixel electrode 30 includes a first horizontal pixel portion 30a, a plurality of vertical pixel portions 30b and a second horizontal pixel portion 30c. The first horizontal pixel portion 30a is connected to the drain electrode 28 and disposed adjacent to and parallel with the gate line 12. The plurality of vertical pixel portions 30b extends from the first horizontal pixel portion 30a substantially parallel with the data lines 24 and are spaced apart from one another. The second horizontal pixel portion 30c is disposed over the common line 16 and connects the other ends of the plurality of vertical pixel portion 30b. The horizontal and vertical pixel portions 30a, 30b and 30c and the drain electrode 28 are formed as one united body. The common electrode 17 includes a horizontal common portion 17a and a plurality of vertical common portions 17b. The plurality of vertical common portions 17b extend from the common line 16 and each of the vertical common portions 17b is disposed between the horizontal pixel portions 30b. The horizontal common portion 17a is disposed adjacent to the gate line 12 and connects the plurality of vertical common portions 17b. The horizontal and vertical common portions 17a and 17b are formed as one united body with the common line 16.
In the above-mentioned structure of the related art shown in FIG. 1, a region D between the gate line 12 and the horizontal pixel and common portions 30a and 17a becomes a non-display area because the electric field generated in the region D is different from that generated in the area between the vertical pixel portion and the vertical common portion. The region D of FIG. 1 cannot display the images, and thus the aperture ratio and brightness of the IPS-LCD device decreases. Furthermore, due to the fact that the IPS-LCD device shown in FIG. 1 has the common and pixel electrodes that are made of an opaque material, the aperture ratio and brightness are further reduced.